1) Field of the Invention
The present application relates to a multilevel optical phase modulator suitable for use in an optical communications system.
2) Description of the Related Art
Recently, practical development of optical transmitters for higher capacity and longer distance of optical transmission systems has been expected. Especially, introducing a real system of optical transmitters adopting an optical modulation method adapted for higher capacity and longer distance has been highly expected. To meet the expectations, optical transmission systems using optical modulation methods such as DPSK (Differential Phase Shift Keying) QPSK (Quadrature Phase Shift Keying), and DQPSK (Differential Quadrature Phase Shift Keying) have been proposed.
FIG. 29 shows a configuration of a multilevel optical phase modulator that transmits QPSK or DQPSK signals. In the multilevel optical phase modulator 2000 shown in FIG. 29, a light source (LD) 2001 generates CW (Continuous Wave) light. An optical splitter 2002 splits and guides the CW light to arms 2003, 2004. Phase modulators 2005, 2006 are provided in the arms 2003, 2004, respectively.
Each of the phase modulators 2005, 2006 includes a Mach-Zehnder interferometer and an electrode formed in an arm waveguide forming the Mach-Zehnder interferometer. The phase modulator 2005 phase-modulates the CW light using data #1 (e.g., DATA 1 and the inversion signal of DATA 1), and the phase modulator 2006 phase-modulates the CW light using data #2 (e.g., DATA 2 and the inversion signal of DATA 2). In DQPSK, data #1, #2 are coded by a DQPSK precoder (not shown).
An optical coupler 2007 couples the modulated signals obtained by the phase modulators 2005, 2006. A phase-shift unit 2008 provides a phase difference of π/2 between the optical signal guided to the optical coupler 2007 via the arm 2003 and the optical signal guided to the optical coupler 2007 via the arm 2004. By the configuration, multilevel optical phase-modulated signals such as QPSK optical signals or DQPSK optical signals are generated and output through the optical coupler 2007.
A dividing unit 2009 partially divides the multilevel optical phase-modulated signal output from the optical coupler 2007 for monitoring. A photodetector (PD) 2010 detects the multilevel optical phase-modulated signal divided by the dividing unit 2009. Specifically, the photodetector 2010 receives the multilevel optical phase-modulated signal from the dividing unit 2009 and outputs an electric signal according to its power.
A phase-shift control unit 2011 adjusts an amount of phase shift of the phase-shift unit 2008 based on the detection result by the photodetector 2010. For example, according to the dithering technology, the control unit 2011 superimposes a low-frequency signal having a relatively small amplitude on the control signal for controlling the amount of phase shift in the phase-shift unit 2008 and controls the phase-shift unit 2008 to optimize the above described amount of phase shift based on the magnitude of the low-frequency signal component contained in the monitor light detected by the photodetector 2010.
Thereby, in the phase-shift control unit 2011, by the feedback of the output signal light as the multilevel optical phase-modulated signal, the phase difference between the optical signal guided to the optical coupler 2007 via the arm 2003 and the optical signal guided to the optical coupler 2007 via the arm 2004 are stabilized to π/2 for good signal quality regardless of change in temperature, deterioration with age, or the like.
JP-A-2007-43638 and JP-A-2007-82094 disclose technologies of stabilizing the phase difference by the feedback of output signal light.
In the optical transmitter shown in FIG. 29, to stabilize the amount of phase shift provided by the phase-shift unit 2008 (“π/2” in QPSK and DQPSK), the output signal light as the multilevel optical phase-modulated signal is taken in as a signal for feedback control. Accordingly, the light formed by superimposing the low-frequency signal on the drive signals of the phase modulators 2005, 2006 and received by the photodetector 2010 contains the multilevel optical phase-modulated signal component, and when the low-frequency signal component is extracted, the multilevel optical phase-modulated signal component itself hinders the correct extraction of the low-frequency signal component.
FIG. 30 shows a comparison between light power (A) detected by the photodetector 2010 and the low-frequency signal component (B) superimposed by the phase-shift control unit 2011. As shown in FIG. 30, intensity fluctuations are caused in the low-frequency signal component detected by the photodetector 2010 due to the multilevel optical phase-modulated signal component itself which is not the low-frequency signal superimposition in the phase-shift unit 2008 and thereby, the correct extraction of the low-frequency signal component is hindered, and consequently, improvement in control accuracy is hindered.
Further, since the monitor light is taken in for controlling the amount of phase shift by dividing the output signal light of the multilevel optical phase modulator 2000, the output signal light power becomes lower.